The objectives of this research are to determine the opsonic requirements of several bacterial species and to assess host factors critical for resistance to serious infections. More knowledge of the surface components of infecting microbes and serum opsonins is needed to develop rational methods for protecting susceptible patients and improving treatment of patients with severe infections. Opsonic requirements of Staphylococcus aureus will emphasize the role of cell wall peptidoglycan as an antiphagocytic factor and the antiopsonic role of capsular material in S. aureus, E. coli and P. aeruginosa will be studied. Opsonic sources will include normal serum,hyperimmune serum, immunoglobulin deficient serum and serums with deficiencies of certain complement components. The role of classical complement pathway and alternative complement pathway for activation of opsonic factors will be evaluated using MGEGTA chelated serum. The patient populations for study will include full-term newborn and premature infants, patients without spleens, patients with immunogobulin and complement deficiencies, and patients with severe bacterial infections. Studies of the opsonic response to pneumococcal vaccines will include patients with nephrotic syndrome and post-splenectomy patients as well as age matched controls. The role of immunoglobulin preparations and fresh plasma for amplification of opsonic function will be tested.